Previously, aluminum supports for lithographic printing plates have been etched in acidic or alkaline aqueous solutions for surface roughening the supports in manufacturing processes thereof. However, treatment unevenness called "streaks" has been liable to occur. This has been said to be attributed to the difference in dissolution speed of surfaces of the aluminum supports (hereinafter sometimes referred to as "aluminum plates") due to the orientation of crystals when the dissolution reaction of the surfaces proceeds.
As to electrochemically surface roughening using direct current, a method as disclosed in U.S. Pat. No. 4,902,389 which corresponds to JP-A-1-141094 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") is known, and as to electrochemically surface roughening using alternating current to form honeycomb pits uniformly, a method as disclosed in U.S. Pat. No. 4,561,944 which corresponds to JP-B-5-65360 (the term "JP-B" as used herein means an "examined Japanese patent publication") is known. U.S. Pat. No. 4,561,944 discloses that the ratio Qc/Qa of the quantity of electricity Qc at the time when the aluminum plate is the cathode to the quantity of electricity Qa at the time when the aluminum is the anode is suitably from 1 to 2.5, and if it is more than 2.5, uniform grain is not formed and energy efficiency is reduced. Further, as to the electrochemically surface roughening using alternating current, JP-A-55-137993 discloses that the ratio Qc/Qa is suitably from 0.3 to 0.95.
JP-A-63-176188 discloses that it is effective to conduct electrolytic polishing treatment after the electrochemically surface roughening treatment.
Further, JP-A-6-135175 discloses that it is effective to conduct chemically etching before and after the electrochemically surface roughening treatment.
As described above, surface roughening of an aluminum plate is carried out by mechanically surface roughening, electrochemically surface roughening, electrolytic polishing, and chemical etching in combination appropriately.
However, when there is a difference in orientation of crystal grains in the aluminum plate to be processed, tatami texture-like stripes called "streaks" as described above tend to occur upon chemical etching. Accordingly, it is necessary to control the heat treatment conditions of the aluminum plate rolling process more severely and as a result, the conventional methods were industrially disadvantageous.
On the other hand, JP-B-57-46436 discloses a process comprising conducting alkaline electrolytic polishing treatment and an aluminum plate that has been mechanically surface roughened, and then, anodizing the plate.
In this process, however, it is neither described nor studied how the alkaline electrolytic polishing treatment acts on the occurrence of the treatment unevenness caused by the orientation of crystal grains.
On the other hand, there is a demand for reduced consumption of the alkaline aqueous solution. For this purpose, it is necessary to establish a high concentration of aluminum in the alkaline aqueous solution and to decrease the amount of the aqueous solution discharged to the outside of a system. However, when the concentration of aluminum in the alkaline aqueous solution is established high, the problems of elevated electrolytic voltage, a tendency to develop the treatment uneveness and formation of oxide films which can not be easily removed have been encountered.